It is known that mechanical energy, usually originating from vibrations in the external environment, can be converted into electrical energy using a variation of the capacitance between two plates.
A known type of energy conversion system comprises a usually external part fixed to a moving support, and a usually internal mobile part making a relative displacement with respect to the fixed part due to its inertia. The internal and external parts comprise interdigitised fingers forming pairs of variable capacitors. The system also comprises a flexible link between the fixed and mobile parts so as to allow their relative displacement and a converter to convert the retrieved mechanical energy into the required energy form, for example electrical energy.
The capacitance of the system varies when the fingers of the combs move. When the combs return to the equilibrium position, an electrostatic force is applied and opposes the mechanical return force that tends to bring the combs back into the equilibrium position, in other words tends to bring each finger of a comb into the middle of the space defined by two fingers of the other comb. Mechanical energy is then converted into electrical energy by electrical charge and discharge cycles on the capacitor combs. The charge may be stored with different types of management cycles. For example, a so-called <<constant voltage>> cycle for which the charge varies, or a so-called <<constant charge>> cycle for which the voltage at the capacitor terminals varies during the conversion cycle.
The efficiency of the conversion from mechanical energy to electrical energy is maximum when the intensity and variation of the electrostatic and mechanical return forces are comparable. In this case, the kinetic energy of the mobile part is practically integrally and instantaneously converted into electrical energy. The mechanical return force is traditionally proportional to the relative displacement between the mobile part and the fixed part. The electrostatic force is constant for operation at constant voltage and is proportional to the separation distance for operation at constant charge.
In devices according to prior art, the electrostatic force opposes the mechanical return force. Furthermore, if the electrostatic force is greater than the mechanical return force, the combs come into contact with each other. Therefore, the electrostatic force has to be less than the mechanical return force if a stable system is to be obtained.
Furthermore, the theoretical conversion efficiency is even better when the variation of the electrical capacitance is high during displacements of the mobile part. Since the value of a capacitor provided with parallel plates is inversely proportional to the distance between the plates, the distance separating the fixed and mobile parts has to be reduced. However, it is preferable to avoid contacts between the fixed and mobile parts to avoid possibly irreversible electrostatic sticking, abnormal mechanical wear and/ or discharge of the variable capacitor.
The article by Scott Meninger, Jose Oscar Miranda, Rajeevan Amirtharajah, Anantha Chandrakasan et Jeffrey Lang entitled <<Vibration-to-electric Energy Conversion>>, IEEE Transactions on Very Large Scale Integration (VLSI) Systems, Vol.9n) 1-2001, discloses such a converter (FIG. 7) provided with interdigitised combs, a first mobile central comb 704 and two second fixed combs 702 on each side of the mobile comb 704. The fingers of the first comb 704 penetrate into a space delimited by two corresponding fingers of one of the second combs 702.
Movements of this converter are limited by the distance between the fingers. Furthermore, this distance is increased by a safety distance to prevent contact. As soon as this amplitude is greater than the allowable movement distance, part of the mechanical energy is lost in contact between fingers. This safety distance has to be relatively large to overcome manufacturing inaccuracies that can also cause contact, correspondingly reducing the possible variation in capacitance and therefore the conversion efficiency.
Furthermore, when at rest the fingers of a comb are in a central position between two fingers of the other comb. Thus, as soon as the movement amplitude is low compared with the distance between two adjacent fingers in the same comb, the conversion efficiency reduces strongly because the movement bringing the fixed and mobile parts towards each other is not sufficient to create a significant change in capacitance.
Furthermore, a single capacitor charge/discharge cycle occurs every time that there is any relative displacement between the mobile comb and the fixed combs. Consequently, the electrical frequency obtained is only twice the mechanical frequency. But in nature and in most instruments, most vibration frequencies are less than 100 Hz, therefore the electrical frequency obtained is too low for the electrical energy obtained to be used in compact high performance electronic systems.
Furthermore, this device is only sensitive to vibrations with a component parallel to a direction transverse to the direction of the fingers.